1. Technical Field
The present invention generally relates to a method and system for producing polymer foam articles. More particularly, the present invention relates to a method and system for producing low-density, high melting point polymer foam articles.
2. Background Information
For some time, low-density polystyrene foam (also referred to as "atactic" polystyrene foam) has been found useful in insulation, packaging, beverage cups and food containers. However, low-density polystyrene products generally have a service temperature limit of about 200.degree. F. Above the service temperature limit, the product will warp and distort. Therefore, there is a general desire for other types of low-density foam that do not suffer such shortcomings.
Polymer resins, such as, for example, poly(ethylene terephthalate) (often referred to as "PET"), polyamides (such as Nylon 6,6), and syndiotactic polystyrene, exist that could be used without suffering such shortcomings. These materials are currently available in the form of solid products, however their high raw material cost renders them useless in many applications for economic reasons. For example, PET is currently widely used to make many recyclable plastic items, such as soda bottles. However, attempts to produce low-density foam articles from polymers with "high" melting points (i.e., greater than 450.degree. F.) has proven to be difficult, and the quality of such foam has been poor. Foam articles made from such polymers have experienced foam cell collapse and/or severe imperfections. Thus, the quality of the low-density, high melting point polymer foam articles has not been close to polystyrene. The problem of collapsing is due to the high foaming temperature such materials demand, for example, PET foams at about 480.degree. F. Using conventional blowing agents at such temperatures results in a high rate of expansion, causing cell wall rupture and allowing the gas to escape. Without gas in the foam cells prior to cooling, the cells cannot support themselves. In addition, many such polymer resins are crystalline in nature, and as such, have an inferior melt strength compared to atactic polystyrene resins. As one skilled in the art will know, melt strength refers to the ability of a material to be stretched at its melting temperature without breaking. The combination of a lower melt strength polymers and higher vapor pressure at the elevated foaming temperature also requires a reduction in the size of the extrusion die opening where the foam exits. Such small die openings lead to a thin gauge foam sheet experiencing severe corrugation at low densities. Corrugation is defines as alternating bands of thicker and thinner sheet in the cross-machine dimension of the sheet. Corrugation is detrimental to the formed product due to thin gauge areas in that product and irregular part weight.
Thus, a need exists for a way to make a quality low-density, high service temperature polymer foam article approaching or achieving the quality of existing polystyrene foam articles.